Michael E. Lamm

The Polymeric Immunoglobulin Receptor Translocates Pneumococci across Human Nasopharyngeal Epithelial Cells

The polymeric immunoglobulin receptor (pIgR) plays a crucial role in mucosal immunity against microbial infection by transporting polymeric immunoglobulins (pIg) across the mucosal epithelium. We report here that the human pIgR (hpIgR) can bind to a major pneumococcal adhesin, CbpA. Expression of hpIgR in human nasopharyngeal cells and MDCK cells greatly enhanced pneumococcal adherence and invasion. The hpIgR-mediated bacterial adherence and invasion were abolished by either insertional knockout of cbpA or antibodies against either hpIgR or CbpA. In contrast, rabbit pIgR (rpIgR) did not bind to CbpA and its expression in MDCK cells did not enhance pneumococcal adherence and invasion. These results suggest that pneumococci are a novel example of a pathogen co-opting the pIg transcytosis machinery to promote translocation across a mucosal barrier.

Cellular aspects of immunoglobulin A.

Publisher Summary Immunoglobulin A (IgA) is the principal class of immunoglobulin in most external secretions. It is made by plasma cells in glands and mucous membranes that face the external environment. The secretory form of IgA, known as secretory IgA, has distinctive antigenic and structural characteristics due to the presence of an extra polypeptide chain, the secretory component. This chapter emphasizes the maturation of IgA during ontogeny; synthesis and assembly of IgA; IgA cell cycle; thymus dependency of IgA; and disorders of IgA production. The chapter discusses the functions of IgA antibodies. Commitment of an immunocyte to the production of IgA is the terminal step in a differentiation process that begins with a cell bearing IgM on its surface. On balance, the evidence favors a direct switch from IgM to IgA. Much of the IgA secreted by the plasma cells in exocrine glands and mucous membranes is dimeric. The fully assembled secretory IgA molecule is the synthetic product of two distinct types of cell, both of which reside locally in the mucous membrane or gland. As is the case with IgG and IgE, the secretion of IgA is thymus dependent. In humans, thymic dysfunction is often associated with diminished IgA. The chapter also presents many interesting and important problems related to IgA that remain to be solved.

IgA Nephropathy: Pathogenesis of the Most Common Form of Glomerulonephritis

IgA nephropathy (IgAN) is being recognized increasingly as a common form of glomerulonephritis with progressive potential (17, 29, 42). Our initial goal here is briefly to consider the definition, clinical features, and major clinical problems of IgAN, and the special characteristics of the secretory immune system, the major source of IgA in the body. Then follows a critical discussion of experimental and clinical observations which provide a conceptual framework for a scheme of pathogenesis based on distinctive features of immunity in mucous membranes and altered regulation of mucosal immune responses due to a defect in mucosal IgG/IgM tolerogenesis. We believe that, in the main, IgAN is an immune complex disease resulting from a poorly controlled mucosal immune response to environmental antigens to which the host is chronically subject. The deposited IgA is likely antibody to viral or dietary antigen. Evidence exists for other mechanisms, which we consider less likely.

A Mucosal IgA-Mediated Excretory Immune System In Vivo

The capacity of mucosal IgA Abs to serve as an excretory immune system in vivo was investigated. Mice expressing a transgenic TCR were immunized intragastrically with the cognate Ag to elicit a vigorous mucosal IgA Ab response. Soon after i.v. challenge, Ag was detected within the epithelial cells of the small intestinal crypts and to a lesser degree within the epithelial cells higher up the villi, paralleling the gradient in expression of the polymeric Ig receptor and the transport of its ligand, oligomeric IgA. Uptake of Ag into the epithelial cells occurred only from the basolateral aspect and only when Ag complexed to IgA Ab could be present in the lamina propria. The results support the concept that local IgA Abs can excrete Ags from the body by transporting them directly through mucosal epithelial cells, using the same mechanism that transports free IgA into the mucosal secretions.

IgA and mucosal defense

The traditional role of IgA antibodies in mucosal defense has been considered as providing an immune barrier to keep exogenous substances, including microbial pathogens, from penetrating the mucosa. In this way infections can be prevented. More recently, studies in vitro and in vivo are providing evidence to suggest that IgA may have additional roles in mucosal defense. For example, during their passage through the lining epithelial cells of mucous membranes en route to the secretions, IgA antibodies may have an opportunity to neutralize intracellular pathogens like viruses. Also, IgA antibodies in the mucosal lamina propria have opportunities to complex with antigens and excrete them through the adjacent mucosal epithelium, again by the same route to the secretions that is taken by free IgA. These latter functions could aid in recovery from infection.

Multiple functions of immunoglobulin A in mucosal defense against viruses: an in vitro measles virus model.

ABSTRACT Three defense functions of immunoglobulin A (IgA), immune exclusion, intracellular neutralization, and virus excretion, were assessed in a measles virus model using polarized epithelial cells expressing the polymeric immunoglobulin receptor and monoclonal antibodies against the viral H and F envelope proteins and the internal N protein. Anti-H IgA was the most effective antibody at preventing infection via the apical surface, i.e., immune exclusion. This IgA was also the most effective at intraepithelial cell neutralization after infection at the apical surface and endocytosis of IgA at the basolateral surface, although an antibody against the internal N protein was also effective. In the intracellular neutralization experiments, confocal immunofluorescence microscopy showed prominent colocalization of anti-H IgA and H protein inside virus-infected cells, whereas colocalization of anti-F and F protein and of anti-N and N protein was much less, in agreement with the neutralization results. Combinations of IgA anti-H, anti-F, and anti-N showed no synergistic effects in intracellular neutralization. In the immune excretion experiments, virus immune complexes with either anti-H or anti-F IgA placed beneath polarized epithelial cells could be transported to the apical supernatant. Anti-F IgA, which was relatively poor at immune exclusion and intracellular neutralization, was the most robust at virus excretion. Thus, the studies collectively demonstrated three different antiviral functions of IgA in relation to epithelium and also suggested that the particular viral component with which a given IgA antibody reacts is an important determinant of the magnitude of the antiviral effect.

Differentiation pathway of Peyer's patch precursors of IgA plasma cells in the secretory immune system.

Abstract Others have shown that Peyers patch (PP) precursor cells can seed the small intestine with IgA plasma cells, whose appearance after cell transfer requires an interval of a week. The location of the precursor cells during this period is the subject of the present investigation in which the fate of radiolabeled murine PP cells was studied after intravenous transfer into primary and secondary recipients. In short-term single-transfer experiments, few radiolabeled PP blasts were found in the small intestine and only a small proportion of these contained IgA. Radiolabeled PP blasts were approximately equally distributed between mesenteric lymph nodes (MN) and peripheral lymph nodes (PN). However, a higher proportion of those which went to MN than of those which went to PN was subsequently capable of settling in the small intestine of secondary recipients and a higher proportion of those arriving in the intestine was found to contain IgA. Thus, MN were distinctly superior to PN as an intermediate site for the maturation of PP-derived IgA plasma cell precursors capable of seeding the small intestine, even when these had been injected intravenously. Treatment of PP cells with antiserum to IgA prior to their injection into primary recipients interfered with the homing of IgA precursor cells to the small intestine of secondary recipients, indicating that the precursors are already producing IgA while still in the PP. IgA-containing, radiolabeled PP cells were found in the subcapsular sinus of MN within 30 min of intravenous injection, suggesting that these cells are capable of extravasating from blood and reaching the lymph within the intestine long before they are ready to remain in the intestine as plasma cells. These results imply that the superiority of MN over PN as an intermediate site for the proliferation and differentiation of PP IgA precursor cells into IgA plasma cells capable of seeding the small intestine is due to the location of the MN in the pathway of circulation of mucosal B cells.

The Human Polymeric Immunoglobulin Receptor Binds to Streptococcus pneumoniae via Domains 3 and 4

Streptococcus pneumoniae (the pneumococcus) is a major cause of bacterial pneumonia, middle ear infection (otitis media), sepsis, and meningitis. Our previous study demonstrated that the choline-binding protein A (CbpA) of S. pneumoniae binds to the human polymeric immunoglobulin receptor (pIgR) and enhances pneumococcal adhesion to and invasion of cultured epithelial cells. In this study, we sought to determine the CbpA-binding motif on pIgR by deletional analysis. The extra-cellular portion of pIgR consists of five Ig-like domains (D1–D5), each of which contains 104–114 amino acids and two disulfide bonds. Deletional analysis of human pIgR revealed that the lack of either D3 or D4 resulted in the loss of CbpA binding, whereas complete deletions of domains D1, D2, and D5 had undetectable impacts. Subsequent analysis showed that domains D3 and D4 together were necessary and sufficient for the ligand-binding activity. Furthermore, CbpA binding of pIgR did not appear to require Ca2+ or Mg2+. Finally, treating pIgR with a reducing agent abolished CbpA binding, suggesting that disulfide bonding is required for the formation of CbpA-binding motif(s). These results strongly suggest a conformational CbpA-binding motif(s) in the D3/D4 region of human pIgR, which is functionally separated from the IgA-binding site(s).

Intraepithelial Cell Neutralization of HIV-1 Replication by IgA

HIV is transmitted sexually through mucosal surfaces where IgA Abs are the first line of immune defense. In this study, we used paired IgA and IgG mAbs against HIV gp160 to study intraepithelial cell neutralization and inhibition of HIV replication. African green monkey kidney cells, Vero C1008, polarizable epithelial cells transfected to express the polymeric Ig receptor (pIgR), were transfected with HIV proviral DNA, and intracellular neutralization mediated by the mAbs was assessed. D47A and D19A IgA, which neutralized HIV in a conventional assay, potently inhibited intracellular HIV replication as assessed by infecting HeLa-CD4-long terminal repeat/β-galactosidase cells (human cervical carcinoma cell line) and CEMx174 cells (human T cell line) with apical supernatant, basolateral medium, and cell lysate from transfected cells. D47A also inhibited the production of virus as assessed by direct assay of p24. In contrast, D47 and D19 IgG, sharing the same V regions, but which were not transcytosed by the pIgR, did not inhibit intracellular HIV replication, nor did D47A and D19A IgA in pIgR− cells, incapable of transcytosing IgA. Confocal immunofluorescence microscopy showed prominent colocalization of HIV protein and D47A, in agreement with the intracellular neutralization data. D10A, which did not neutralize HIV in the conventional assay, and irrelevant IgA did not show intracellular neutralization or colocalization. Control studies with two kinds of conditioned medium confirmed that HIV neutralization had indeed occurred inside the cells. Thus, during its transcytosis through epithelial cells, HIV-specific IgA can neutralize HIV replication.

Inhibition of microbial IgA proteases by human secretory IgA and serum

Microbial IgA proteases cleave human serum IgA1 immunoglobulin, but human secretory IgA is resistant to hydrolysis. We have found this resistance to be due to an inhibition of protease activity that is mediated by the Fab region of secretory IgA. The IgA proteases of the genus Neisseria are more sensitive to inhibition than is the protease of Streptococcus sanguis. There is also a serum inhibitor of Neisseria proteases that co-chromatographs with IgG. Monoclonal (myeloma) human IgG proteins and plasma protease inhibitors such as alpha-1-antitrypsin and alpha-2-macroglobulin do not inhibit. Human sera do not contain inhibitor to S. sanguis protease activity. We conclude that microbial IgA proteases are subject to inhibition by IgA in secretions and IgG in serum, and this activity is most consistent with being an anti-enzyme antibody. The insensitivity of S. sanguis IgA protease to inhibition is unexplained but provides further evidence that the IgA proteases are structurally diverse.